The present invention relates to a hydraulic levelling control system for a loader type vehicle and particularly to an improved hydraulic system for controlling the attitude of a material handling unit supported on a boom, during raising and during lowering of the boom.
Loader type vehicles generally include a boom pivotally carried on the vehicle with a material handling unit pivoted on the free end of the boom. Generally, the boom is raised and lowered on the vehicle by a boom cylinder having fluid supplied thereto through a boom control valve while the material handling unit is pivoted on the end of the boom through a unit cylinder having fluid supplied thereto through a unit valve. Fluid is generally supplied to the respective control valves from a reservoir through a pump.
In the absence of any self-leveling function, it is necessary for the operator of the loader type vehicle to operate both the boom valve and the unit valve one with each hand or foot, to maintain the material handling unit level while raising or lowering the boom. This operation is not only difficult but also requires the strict attention of the operator.
Hydraulic leveling systems have heretofore been made such as disclosed in U.S. Pat. No. 3,563,137, in which fluid exiting from the rod end of the boom cylinder during raising of the boom, is passed through a valve type flow divider that directs one portion of the inlet flow to the piston end of the unit cylinder while dumping a remaining portion of the inlet flow to drain, to level the unit during raising of the boom. This patent also discloses an embodiment in which the boom control valve was modified to provide a boom-raise, unit-leveling position and a boom-lower, unit-leveling position, in addition to the customary boom-raise and boom-lower positions. In that embodiment, the boom control valve was operative to pass fluid existing from the piston end of the boom cylinder during lowering of the boom through a second flow divider valve arranged to direct one portion of the flow to the rod end of the unit cylinder and to dump the remaining portion to drain. Valve type flow dividers divide flow from an inlet into separate streams at two outlets to in accordance with the pressure drop through two orifices and can operate reliably only when the pressure at the inlet is substantially greater than the pressure at either outlet. During lowering of the boom, a portion of the fluid from the piston end of the boom cylinder is passed to the rod end of the bucket cylinder. The area of the piston end of the boom cylinder is usually very large as compared to the area of the rod end of the unit cylinder and, under some operating conditions, the pressure at the piston end of the boom cylinder is too low as compared to the pressure required at the rod end of the bucket cylinder, to reliably move the bucket when the latter is loaded.
U.S. Pat. No. 5,447,094 discloses a leveling circuit using a valve type flow divider for controlling leveling of the bucket during raising of the boom and a motor type flow divider for controlling leveling of the bucket during lower of the boom. While this hydraulic leveling system operates during both raising and lowering of the boom, the motor type flow divider significantly increases the cost of the leveling system. Further, motor type flow dividers divide fluid in a fixed ratio and are not adjustable to change the fixed ratio to accommodate different size boom and unit cylinders such as are used in different loader-type vehicles.